This invention relates to techniques for determining location and more specifically to techniques that utilize wireless communication devices such as cellular base stations and WiFi access points.
The NAVSTAR Global Positioning System (GPS) developed by the United States Department of Defense uses a constellation of between 24 and 32 Medium Earth Orbit satellites that transmit precise microwave signals, which allows devices embodying GPS sensors to determine their current location. Initial applications were predominantly military; the first widespread consumer application was navigational assistance.
With the explosive growth in mobile communications devices, a new wave of location-based applications is emerging. These applications are characterized by a requirement for device-centered maps. One example is a form of “Yellow Pages” in which a map centered at the location of the mobile device presents user-selected establishments (e.g., barber shops) in situ. Another example would be an application enabling one to locate, on a device-centered map, members of his or her social network.
In a strong signal environment (such as outdoors), GPS is a reliable and accurate source of the location information essential to enable a device-centered map to be served across a network to a mobile device. The location of the client device is determined by using the time-difference of arrival (TDOA) of signals transmitted from a GPS satellite to a client device. The location of the client device is determined by using TDOA information between the client device and at least four GPS satellites, as there are four unknowns that need to be determined (X, Y, Z coordinates and the local time of the client device). For a 2-dimensional location, TDOA information from three GPS satellites are used as the Z coordinate is assumed to be on the surface of the Earth.
In contrast to the relatively-robust strong-signal operation, satellite-based positioning systems such as GPS work poorly if at all in weak-signal environments such as indoors. The received satellite signal is relatively weak and thus greatly attenuated by structures. Conventional GPS receivers, even when operating in 2D mode (using just three satellites and, most commonly, a pseudo satellite located at the center of the earth) do not have sufficient sensitivity to provide reliable and accurate location information in weak-signal environments. FIG. 1 describes the well-known difficulty in obtaining a GPS fix at indoor locations. The path losses caused by walls, windows, roofs, floors, and other structural components weaken the already relatively-weak unobstructed received signal strength such that the receiver cannot acquire sufficient satellite signals for a position fix. The path losses and other factors such as multipath ambiguity prevent the determination of TDOA information.
As a result, various non-satellite-based positioning systems have been developed to aid in position determination in weak-signal environments. For example, the received power from a local cellular base station for a mobile device is generally considerably greater than the power of a received satellite signal as the base station may be just hundreds of yards away whereas the GPS satellites are orbiting thousands of miles away in space. Cellular-based positioning systems such as shown in FIG. 2 thus offer an improvement over satellite-based systems for indoor positioning applications. Since the precise location coordinates of individual cellsites in a cellular system are known, the estimated location of client devices equipped with a cellular radio can be determined, especially since signals from cellsites were designed to provide reliable coverage in indoor and urban canyon environments. A cellular phone scans the cellular frequency band for available cellsites. The received signal strength of each cellsite is determined. This is commonly referred to as RSSI (received signal strength indication). The RSSI signal is used primarily to determine the best cellsite to be used by the cellular phone to provide the communication link between the cellular phone and the cellular system. Prior art location technology utilizes the RSSI information between a client device and cellsites to approximate the location of the client device. Similarly, there are prior art examples which provide location information of a client device which use RSSI signatures from other wireless communication devices such as WiFi access points (APs). However, determining the physical distance between a transmitter and a receiver via an RSSI signal is not precise because it varies widely with the particular RF environment.
There is need in the art to provide the location of a client device which has improved accuracy over RSSI based systems even in indoor and urban canyon environments.